In normal day-to-day human use of the telephone, the ear is relied upon to detect when a called party has answered a telephone call. The called party's pick-up is normally manifested either by a termination of the ring-back signal and a response such as "hello", or a termination of the ring-back and a period of silence. In PBX equipment, and devices for supervising calls from a central office to subscriber lines connected to the office, the supervisory signals present on the called party subscriber line may be used to detect a pick-up condition.
However, in more recent times, automatic dialing equipment has been developed, giving rise to the need of a device which may be placed on a calling party subscriber line and detect pick-up of the telephone set at a called party subscriber line, when the call is routed through any number of intermediate offices, including the toll network. Under such conditions, no supervisory signals are available at the calling location. Thus, detection of pick-up may be made only by the audio information on the line. Since the North American telephone network, and telephone networks throughout the world exhibit a wide variety of signal conditions, the audio information presented to the calling party subscriber line varies substantially. This variation is further exacerbated by the variety of noise conditions which may appear on any given call and the variety of human voice spectra, and intensity with which answering parties will speak, into the telephone set. Also, many people are in the habit of waiting for the party initiating the call to speak first.
Also, as is known to those skilled in the art, local offices at various locations in the country provide a variety of ring-back signals to the calling party. This is also true of some calls connected through PBX and Centrex equipment.
In the past, certain automated calling devices have detected special conditions upon pick-up. For example, automatic dialing intelligent modems are often configured to wait a certain period of time after the dialed number has been pulsed out for an appropriate carrier signal from an answering modem. If such is not received within a predetermined period of time, the modem will default to another mode of operation. It is also known to detect the frequency spectrum of the slow busy signal used in most of the United States.
Still more recently, automated dialing equipment has been used to initiate a large number of calls which may be handled by fewer persons than the total number of calls in progress. Such equipment is typically configured to include automatic dialers, which often dial sequentially through all the numbers in a particular exchange, and which initiate a "conversation" through the use of a prerecorded greeting and/or set of questions. These devices are used for telephone canvasing, advertising, and attempts to make direct sales to the called party.
While some subscribers have found these devices to be rather obnoxious, they are becoming firmly established in the stream of commerce and are quite useful. Automatic dialing equipment has been found to be particularly useful in contacting large numbers of people in both military and civil emergencies. It is in such a device that the present invention is primarily intended to be used.
As noted above, the basic problem with successful operation of such a device is that the machine must be configured to provide apparatus which, using solely audio information, can detect the pick-up of the called telephone set and proceed with the automated portion of the conversation. Normally, if the responses of the called party are satisfactory (which usually means that the telephone is not forthwith placed back on-hook), a human operator will intervene and continue with the business at hand.
Prior art systems for detecting pick-up in automatic dialing equipment have almost universally relied on signals processing schemes using automatic gain control to amplify the audio signals on the telephone line during periods of relative intensity of sound on the line. Average statistics for the spectral content of the human voice have been used to design filters which attempt to detect the presence of a human voice on the line. A common denominator of such schemes has also been that the parameters of the audio signal on the line at the calling party's location by which the decision whether to treat the call as answered are made, are fixed. Thus, audio conditions on the line at the automatic dialing equipment which do not fall within the static mathematical model of what sort of audio signal indicates a pick-up condition, and what sort of audio conditions indicate absence of pick-up, cannot be learned in real time by the apparatus. Furthermore, the reliance upon automatic gain control in the prior art makes such systems much more susceptible to responding to spurious noise present on the line.
While the statistics of average conditions indicating pick-up are useful and have been used to implement pick-up detectors which operate in a generally satisfactory manner, it is well known to those skilled in the art that there are several conditions under which the devices will make an improper decision. Indeed, within the industry there is a collection of known telephone numbers for which automatic pick-up detection is quite difficult, or has not been satisfactorily accomplished in the past. Thus, there is a need within the art for an automatic pick-up detector which will respond reliably, not only under normal conditions of the audio signal on the line at the calling location, but which will also respond to a wide variety of unusual signals to automatically detect that the called party has gone off-hook.